Change units , and respect the scale of the scene?
When you change the unit preset to centimetres you are changing the unit scale. The unit scale determines how big a unit is, a scale of 1.0 means one unit is one metre, and a scale of 0.01 means the same one unit is 0.01 metre or one centimetre.



You don't need to change the unit scale to be able to enter centimetre measurements, you just need the object size to be the correct scale compared to other object in your scene, this would mean scaling down small objects that import too large or adding .0 before any measurements you want to be in centimetres.

To move an item using unit measurements, press = to start advanced entry mode before you enter the amount. That is GX=30CM will move the object 30 centimetres on the x axis.